This invention relates to electrostatically shielded transformers of the type particularly used as isolation transformers to isolate sensitive electrical and electronic equipment from the voltage variations caused by electromagnetic and electrostatic interference, the interference signals being superimposed on the voltage signal supplied with power lines by public utilities. An isolation transformer, by itself, makes no attempt to regulate the amplitude of the supplied voltage signal, but does attenuate interference signals that generally are of a higher frequency and often of a transient nature.
Interference can be caused by equipment belonging to other users of the power line, by electromagnetic or electrostatic fields from many kinds of equipment including electric welding machines, diathermy machines, automotive ignition systems, by lightning, and by various discharges in the power line equipment.
The isolation transformer particularly attenuates common-mode interference. Variations in voltage caused by common-mode interference are equal in amplitude and phase with respect to ground on both lines of the power line pair. These variations are not transmitted from primary winding to secondary winding by normal inductive transformer action because there is no variation in voltage across the primary winding. They are, however, transmitted from primary to secondary in direct proportion to the capacitance between primary and secondary windings. The common-mode interference currents, being alternating currents, flow through this capacitance and eventually back to ground through the load when grounded; or through various capacitances between parts of the secondary winding and ground, and through the capacitance between the load and ground. This is, of course, objectionable.
Isolation transformers using present art place a metallic shield between primary and secondary windings, and ground the shield. Common-mode interference currents will then flow through the primary-to-shield capacitance to ground, providing isolation for the secondary winding and its load from the common-mode interference on the primary.
The primary and secondary windings are fabricated separately. They are then assembled with a multiplicity of ferromagnetic laminations that make up the core. The laminations may be stacked individually, some portions passing through the centers of the windings, or they may be preassembled in pieces that are placed through the centers of the windings and held in place by metallic bands. In any case the primary and secondary windings encircle some portion of the core, passing through at least one opening in the core. The opening is called a window.
The metallic shield is then inserted between the primary and secondary windings, the shield extending both inside and outside the windows in the core. Often metallic end bells around the windings where they are not within the windows in the core. Typically four bolts passing through holes in the laminations and the end bells hold the transformer together.
Generally the fit between the metallic shield and the faces of the windows in the core is poor. In the interest of economy, loose dimensional tolerances are used for the core, the windings, and the shield. The shield must be fairly rigid (typically 2.5 millimeters thick) so that it can be inserted without breaking up or being deformed. A thick shield is undesirable because as the spacing between primary and secondary windings increases, leakage inductance increases causing a degradation in no-load to full-load voltage regulation.
In most core configurations, the faces of the windows comprise the edges of a multiplicity of stacked laminations. The shield butts up against what amounts to a saw-tooth surface. The poor fit between the metallic shield and the faces of the windows in the core causes gaps through which unintercepted electrostatic field lines extend between the primary and secondary windings. These gaps cause a capacitance, of small but important magnitude, to exist between primary and secondary windings. Furthermore the capacitance is highly variable between specimens assembled on the same production line.
This residual capacitance directly between the primary and secondary windings has been called interwinding capacitance by manufacturers of isolation transformers. Although this term does not appear in standard electronics dictionaries, it is useful and descriptive and will be used here.
Interwinding capacitance is determined by applying a measured common-mode, alternating current voltage between the shorted primary winding and ground. The voltage between the secondary and ground across a known impedance is measured, with the secondary winding shorted out, and the shield grounded. The capacitance is then calculated with elementary circuit theory, using the two voltage measurements, and the known values of applied frequency and load impedance.
Isolation transformers using present art are rated according to interwinding capacitance. The lower the capacitance, the better the isolation, and the higher the price. Typical quality classes are 0.005, 0.001, and 0.0005 picofarads. There is a need and a market for isolation transformers with much lower interwinding capacitance.